This invention is broadly related to the field of electronic musical instruments, particularly electronic organs or other electronic musical instruments having a keyboard such as electronic pianos, accordions and the like. The term "organ" as used throughout the specification and claims is intended in a generic sense to include these other electronic musical instruments. In addition, reference to the actuation of key switches or coupler switches and the like is intended to cover the actuation of such switches by whatever means may be employed, such as directly by action of the musician's fingers or indirectly through intervening levers, apertures, switch closings, touch responsive switches, multiplex circuits, etc.
In the design of typical electronic organs today, a system known as a top octave frequency synthesizer system (TOS) has been developed which overcomes the need for using a large number of expensive stable oscillators in the organ. Instead, a single stable oscillator is used to provide the tones for the top octave of the organ. Divider circuitry then is employed to generate all of the other related tones, and tuning of such an organ becomes a relatively simple matter since only a single oscillator or a small number of oscillators are used in the organ.
While top octave synthesizer systems theoretically can use a single oscillator for an entire organ this has not proved to be practical. One disadvantage of employing a top octave synthesizer is that because of the close interrelationship of all of the divided-down frequencies it is possible to obtain phase reinforcement or phase cancellation of harmonics of different tones, which results in very unnatural quality musical production by the organ. To overcome these disadvantages, a number of different top octave synthesizers have been utilized in an organ; so that different notes for different octaves in the different manuals of the keyboard are produced by different top octave synthesizers. If such synthesizers are dedicated to a particular block of keys or a particular part of the organ, however, it still is necessary to use a relatively large number of synthesizer circuits in the organ.
In the system disclosed in the co-pending patent application Ser. No. 867,907, a limited number of different top octave synthesizer circuits are used, each of which is capable of producing any note in the organ. The assignment of different root notes to these different top octave synthesizer circuits is effected under control of a latch signal synchronized with the serial digital data representative of key closures used in the multiplex system with which the synthesizer circuits are used. In such a system, where the number of different top octave synthesizer circuits are less than, equal to or only slightly more than the maximum number of root notes which normally are played by the organ, the assignment of a previously unassigned top octave synthesizer circuit ordinarily can be controlled by a suitable logic circuit which senses whenever a particular top octave synthesizer circuit is idle, waiting for a new note to be assigned to it. The determination of whether or not a synthesizer is idle cannot be ascertained merely by sensing the key closures associated with the initiation and termination of the note produced by any given top octave synthesizer. This is because there generally is a decay of the produced tone which extends the tone in attenuated fashion (with increasing attenuation) after release of the key, the closure of which initially produced that tone.
Because this characteristic of permitting a top octave synthesizer to continue to produce a tone in an increasingly attenuated fashion after release of the key which initiated that tone, it is possible in some circumstances, to create a demand for more root tones in the system than there are empty or wholly unassigned top octave synthesizer systems available to produce the tones demanded. Of course, one solution is to provide a number of top octave synthesizers which is greater than the maximum number of tones which could be produced at any time in the organ whether the tone production results from the actual playing of a key or the decaying tonal characteristics after a key is released. This approach, however, is wasteful of synthesizer circuits and substantially increases the cost and complexity of the circuitry in the organ, and its ultimate price to the customer purchasing such an organ.
To minimize the number of top octave synthesizer circuits needed and to continue to permit the organ to produce the most natural sounding musical characteristics, it is desirable to reassign a top octave synthesizer circuit to a new note only if such a top octave synthesizer is (1) operating on the decay tonal characteristics of a note indicating that its actual playing has been terminated and (2) if such a top octave synthesizer circuit is the one in the system which is the farthest into its decay, that is, the one with the most highly attenuated tone output.